Cyril Carr's plan build article - 30/9/10
In aviation’s short but fruitful genealogy, the story of the autogyro is one of a largely overlooked and undervalued hybrid belonging to the inter-war years. By splicing the fuselage and control surfaces of a fixed-wing aircraft and the flying surfaces created by unpowered rotors, the Spaniard Juan de la Cierva created a machine that combined S/VTOL performance with the mechanical and operational simplicity of a conventional aeroplane. Though still in small-scale production today, the autogyro was a commercially short-lived breed, hastening as it did the development of its evolutionary successor, the helicopter.
The design for this model’s fuselage is based on the two-seater Cierva C30, a mid-30s autogyro. With its front-mounted engine and free-spinning rotors on their short stub-wings, the Kestrel makes an unusual sight in the air and has delightful flying characteristics. A .25 cu. in. two-stroke engine gives the model a very good rate of climb, and it’ll perform loops and barrel rolls with ease. On full power, you can even make fairly tight climbing turns. Its stability is such that it will fly hands-off, and if the engine stops it can be autorotated to the ground. I first flew this model about 10 years ago, and despite some severe prangs it has taken the knocks well and given splendid service throughout.
The fuselage is built around a 1/8” balsa box whose construction is very straightforward. Begin by cutting the sides, base and top, and the formers F1, F2 and F3. Then, having marked the positions of the formers on the base and sides, glue F1, F2 and F3 onto the base and add the two sides. When this assembly has set, glue the rear half of the fuselage sides to F3 and the base. Don’t forget to reinforce the join between the engine bulkhead Fl and the fuselage sides with 1/4” square balsa.
Next, insert the control run tubes for the rudder and elevator and also for the aerial, before gluing the top of the box in place. When all the glue has hardened, remove the section of the fuselage top that lies beneath the wing and cut out the cockpit. Formers F17 and F16, the fuselage reinforcement section and the wing fixing-plate, can then be cut and glued into position.
At this point, you’ll need to make a servo fixing-plate to suit your radio gear and install it as shown on the plan, before fitting formers F4, F5, F6 and planking-in with 1/8 sheet. Note that F6 is faced with 1/32 ply, and its joint with the fuselage box top should be reinforced with nylon cloth and balsa cement as this takes the lifting force of the wing.
Formers F7, F8 and F9 are glued in position, followed in quick succession by the tailplane. The upper fin is made from 3/16 soft balsa and fitted squarely to the fuselage box, whereupon the rear fuselage structure can be formed using 3/16 stringers. After gluing formers F11 to F15 in place, install the lower fin and the lower fuselage stringers as before.
Moving to the front of the model, fit the nose block and the ply undercarriage support F18 before attaching the fuselage side stringers, and completing the cockpit shape with the two F19 formers.
Reinforce the points where the control runs exit the fuselage using small pieces of 3/16 square balsa.
The tailwheel is carried on 12-gauge wire running in a brass tube, but to give some suspension, the main wheels are mounted on 8-gauge wire that is attached to the engine mount. Remember, when fitting the engine mount, to incorporate two degrees of right-hand thrust bias to offset torque.
The lower skins of the distinctive stub-wings can now be cut to shape and marked with the positions of the ribs, spars and trailing edge. After gluing the trailing edge in place, add the lower spar and then the ribs, noting that the root ribs R1 are set at 10 degrees for dihedral. Next, glue in the top spar and the leading edge; after they’ve hardened you can add the top skin. The wings then need to be joined together to give the required 20 degree dihedral. When the wing assembly has set, cut slots in the lower skin for W1 and W2 and wrap the wing join with 11/2” wide nylon cloth.
As you’ll see from the plan, the rotor shafts are made from 10 gauge piano wire mounted in 1/4” ply, which is drilled and slotted to give a secure fit when the rotor shaft is glued into position. Fit W3 and W4, and when set plank-in with 1/8” balsa. Install the wing fixing dowels at the leading edge and the wing bolt at the trailing edge.
The hub discs are made from 1/8 ply or liteply, while the 12 x 2” rotor blades are cut from hard 1/8 balsa. You don’t need to worry about an aerofoil section on the blades - just sand the leading and trailing edges round and leave the rest of the surface flat. To construct the rotor assembly, begin by marking the centre of the hub and the blade positions on a flat building board. Then place the lower hub disc over the centre point, overlap the blade roots as shown in the plan, and glue them together with balsa cement.
Glue the second disc to the top and weight the whole assembly down before raising the centre point of each blade tip 10mm above the board to give the blades a slight dihedral.
When mounting the rotors, I’ve found that the centres of nylon aileron bellcranks make the most convenient bearings. Simply drill the discs so that the centres are a tight fit, and you’ll find that they slip nicely over the 10-gauge shafts with a steel washer fitted above and below the bearings (a dash of 3-in-1 oil reduces any stiction). Make the rotors secure with three collets, two fitted above the assembly, and one below.
You’ll notice that the plan includes an alternative rotor configuration in which the wing is replaced by a single-shaft system that uses the same fuselage mounting. With this set-up, you could experiment with a teetering single rotor of 36” diameter, or a contra-rotating arrangement of 30” diameter. I’ve tried a contra-rotating set-up myself, though I did find that roll control was more difficult.
When you come to install the motor, you may find it necessary to make small adjustments to the shape of the nose and fuselage sides to accommodate the silencer. With these done, fit the engine, throttle control tube and the fuel tank / pipes - the large hole in the fuselage where the wing fits should make this easy enough. Finally, cover the model with Solartex or film before hinging the control surfaces and connecting the control runs. I’ve found that it’s easier to cyano’ the rotors back together after a prang if they’re not covered, so I suggest that you simply balance the blades and spray them matt black with some coloured stripes near the tips.
While a good .25 motor gives the Kestrel ample power, you may wish to install a .32 or even a .40cu. in. four-stroke for added zest. Whatever you use, you’ll need to adjust the centre of gravity to the position shown on the plans, i.e. 13/4” back from the l.e.
Take-off is performed exactly as for a fixed-wing model, after which you can climb away and feel the effects of the controls. The model can be flown very slowly by raising the nose and adding power.
In a correctly weighted example, the stall is quite interesting to watch - try it with the engine at idle. Stall recovery is very easy - just open up and push the stick forwards. If the engine actually stops in the air, push the stick forwards to maintain air speed and fly a dead-stick circuit flaring in the usual way about one foot above the ground (Note that if a rearward C of G is used and the engine stops in flight, the model’s otherwise benign stall characteristics may be impaired).
Loops and rolls can be performed from level flight, and if you lose control you only have to neutralise the sticks to make a recovery. The Kestrel’s as easy as any trainer to fly, especially in the landing when you simply hold a very nose-high attitude on the approach and control the height with power. Happy hovering!
Model type: Semi scale autogyro
Designed by: Cyril Carr
Rotor / wingspan: 48'' (total)
All-up weight: 3.25 lb
C of G: 1.75” back from l.e.
Fuselage length: 34''
Engine (rec’d): .25 – .32 cu. in. two-stroke
Control functions: Rudder, elevator, throttle
C of G issue By derek critcher 2
by derek critcher 2
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